A lightning arrester is a device which is placed between a phase and ground in a high tension line, and which serves to limit the amplitude and the duration of atmospheric over-voltages (surges due to lightning and to induction phenomena in the conductors), or to temporary electric overvoltages on the grid (operating surges).
The functions of a lightning arrester are firstly to withstand normal operating tension on a permanent basis, and secondly to pass the high discharge current which appears during a temporary surge, thereby protecting line apparatuses (transformers, . . . ).
These functions are generally provided by a core made of a material of the varistor type and based, for example, on zinc oxide (ZnO) whose electrical resistivity is highly nonlinear as a function of applied voltage.
This nonlinear characteristic enables such a lightning arrester to pass:
a low current (e.g. about 0.5 mA/cm.sup.2) when the operating voltage is applied on a permanent basis to the lightning arrester, which then presents a very high resistance; this current is essentially capacitive in origin since the relative permitivity of such varistors is very high; or
a high current, which may be as high as several tens of kiloamps, when the applied voltage reaches a trigger threshold above which the resistance of the varistor becomes very low.
Various lightning arrester structures are known which implement a central core comprising a stack of a plurality of cylindrical pellets made of varistor type material, together with two metal end fittings which are in electrical contact with the pellets, e.g. via springs.
In a prior art manufacturing method described in U.S. Pat. No. 4 656,555, a stack is made of the pellets and the end fittings including very strong interposed springs by applying considerable compression, up to as much as severan tens of kg/cm.sup.2, along the axial direction of the stack. Thereafter a winding of resin-impregnated glass fiber filaments is made around the assembly compressed in this way. The winding is made at a very small angle relative to said axis. The fiber is tensioned successively over a shoulder on one of the end fittings then over the corresponding shoulder on the opposite end fitting. Such a winding is mechanically very strong in the longitudinal direction and is intended to counterbalance the pre-stress induced in the stack of pellets prior to and during winding. This strength for withstanding such longitudinal prestress necessarily implies that shear stresses are set up at the interface between the stack of pellets and the envelope constituted by the winding of filaments under tension, in particular when temperature variations occur, since the materials constituting the assembly have very different mechanical characteristics. The locations where the envelope leaves the stack can then become seats of partial electrical discharge or of sufficient arcing activity to degrade or even completely short circuit the assembly in the more or less long term.
The object of the present invention is to implement a method of manufacturing a lightning arrester which provides a product that is more reliable than the prior art lightning arrester.